The present invention relates in general to the fabrication of circuits of the electronic, optronic, and/or optoelectronic type on thin layers of semiconductor material such as silicon. More precisely, in a first aspect, the invention relates to a method of fabricating a plurality of chips, each chip comprising at least one circuit. In a second aspect, the invention also relates to a method of fabricating a support allowing the chip fabricating method to be implemented.
In general, the term “cutting” means an operation consisting in separating individual chips from each other.
The term “chip” means a module comprising one or more circuits.
The term “circuit” means any type of microelectronic, optoelectronic, optronic, and/or optoelectronic circuit.
The term “layers” as used here preferably means thin layers, of thickness which may be of the order of 0.1 micrometers (μm) to 10 μm, for example.
Finally, the circuits which are produced in this way are typically produced in repetitive manner, forming a plurality of identical circuits on a thin layer, which allows a plurality of identical chips to be formed in corresponding manner.
In general, methods of fabricating chips are already known. A first type of known method, it will be recalled, consists in producing chips directly in the surface portion of a massive substrate of semiconductor material (for example silicon or a Group III-V material). Once the chips have been formed, the substrate is cut through its entire thickness to separate the chips from one another. Cutting is carried out by scribing the substrate. When scribing, in general, a score line is initially scribed, after which the substrate is broken along the scribed line. One limitation with that type of method is that chips cannot be produced on a thin substrate. The massive substrates from which the chips are formed are relatively thick (of the order of at least one hundred micrometers—for example 725 μm for a substrate that is 200 millimeters (mm) in diameter); such thicknesses may prove to be too large for certain applications.
Non-limiting examples of such applications for which massive substrates are unsuitable are:                the fabrication of chips comprising light-emitting diodes (LEDs), since too thick a substrate on which the chip is formed may influence the optical behavior of the circuits, for example, when the substrate absorbs part of the light; and        the fabrication of chips that are required to have a degree of mechanical flexibility from substrates of the monocrystalline silicon type, which is a rigid, frangible material; thus, to provide the chip with the required flexibility, the substrate has to be thinner than the massive substrates used.        
Clearly, it is possible to thin the back face of a massive substrate of the type mentioned above, into the surface layer on which the chips are formed (the back face of the substrate being defined here as the face opposite to the “front” face of the substrate, which is the face that carries the chips). By way of example, such thinning can be carried out by chemical etching of the back face of the substrate, or by mechanically attacking the back face. However, substrate thinning is necessarily limited as the substrate must retain a certain thickness (at least on the order of 50 pm) in order to retain an acceptable mechanical strength.
Further, applications such as those mentioned above, which require chips to be formed using a thin layer, remain difficult to access even after thinning. Thus, it can be seen that there are limitations associated with the first type of known method. It should also be noted that scribing chips may be associated with problems (e.g., flaking of the scribed substrate), which constitute a further limitation to that type of method.
A second type of method of producing chips on a substrate is also known. In that second type of method, the following steps are carried out:                creating chips on a layer of semiconductor material, the layer being integral with a substrate;        transferring a layer including the chips from the substrate to a support;        forming individual chaps by cutting the layer in accordance with a predetermined cutting pattern.        
It is specified that the term “transfer” is understood it this text as an operation implying a bonding between a donor wafer (which can be referred to as the “top”) and a receiver wafer (which can be referred to as the “base”), and a subsequent removal of excess material from the top wafer after bonding. More precisely, the “bonding” referred to for defining the “transfer” is a bonding based on molecular adhesion between two surfaces which have an extremely low roughness (typically on the order of a few angstroms, or a few tens of angstroms). The well known SMART-CUT® method is an example of such a transfer method.
It should also be mentioned that the term “predetermined pattern” means a pattern which has been manufactured to define the desired cutting lines. Thus, the chips are formed in the layer of semiconductor material before transferring the layer to a receiving support.
The layer of semiconductor material may be a “thin” layer, i.e. with a. thickness of the order of 0.1 μm to 10 μm. The predetermined pattern typically corresponds to a grid with square or rectangular compartments, with the grid lines defining the boundaries of the chips.
Cutting is typically carried out by scribing the thin layer and, optionally and at the same time, the support with which the layer has been rendered integral.
The thin layer can be transferred to the support using any type of technique that is known per se. In particular, it is known that this transfer can be made by fracturing a zone of weakness provided between the thin layer to be transferred to the support and a substrate with which the thin layer is initially integral. This zone of weakness may be formed before fabricating the chips as mentioned above, or subsequently, if desired.
It is also possible that only certain steps in the chip fabrication are carried out before forming the zone of weakness, with the other steps for fabricating the chips being carried out after forming the zone of weakness. This zone of weakness can, for example, be produced by implanting one or more atomic and/or ionic species, as is the case with SMART-CUT® techniques. The zone of weakness can also be obtained by controlling the bonding energy between two layers presenting a common interface which defines the zone of weakness—these are broadly referred to as “detachable” substrates—wherein a layer can be transferred by applying a stress (mechanical and/or thermal in particular) at the level of the weakened interface. For example, a zone of weakness can thus be formed by creating a porous zone between two layers of substrate, or by producing a reversible bond between the two layers. Other methods can also be envisaged.
It should also be mentioned that it is also known to transfer a thick substrate onto the support, and then thin the transferred substrate via its back face (BSOI® or BESOI® type techniques). That produces a thin layer transferred onto a support. After transferring the thin layer and its circuits to the support, the layer is cut to form individual chips. That type of known method can produce chips having layers with a substantially reduced thickness. It can also allow chips to be formed an a support made of a material which is different from the material of the substrate in which the chips have been produced (the material of the thin layer), and it can have the desired nature and specific properties. However, the disadvantages mentioned above regarding scribing and in particular flaking remain. These disadvantages are more pronounced when scribing layers of small thickness. Thus, the present invention seeks to overcome these problems.